deMartin, Brian

Seismic structure of the Yellowstone Hotspot

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Known primarily for its world renown display of
geysers, hot springs, and fumaroles, Yellowstone National Park is also
characterized by recent, (2.0, 1.2 and 0.6 Ma) cataclysmic silicic
volcanism. Numerous postcaldera silicic and basaltic flows, some as
recent as 70 ka, emphasize Yellowstone's youthful volcanism. Large
normal faults associated with Basin-Range extension, wide spread
earthquake swarms, exceptional crustal deformation of up to 1 m of
caldera uplift from 1923 to 1984, reversing to subsidence of up to 15
cm by 1995, and high thermal fluxes of up to ~ 2000 mW m-1 confirm the
fundamental role of active igneous and tectonic processes at
Yellowstone. In order to understand the surface manifestations of
subsurface igneous and tectonic phenomena numerous studies have
attempted to determine the seismic structure of the Yellowstone region
(e.g. Lehman et al., 1982). The most recent study (Miller and Smith,
1999) has derived a three dimensional P wave velocity model for the
region below the Yellowstone caldera using a total of 66, 627 P travel
times. Five low-velocity zones were identified in the model (see
Figure) and were well correlated with dominant surface features (e.g.
Mallard Lake resurgent dome and Hot Springs Basin).

The first low-velocity zone, associated with the caldera, coincides
with a prominent 60 mGal negative Bouguer gravity anomaly. P velocities
beneath the caldera average 5-5.6 km s-1 compared to the 6-7 km s-1
modelled outside of the caldera boundary within the thermally
undisturbed basement rocks. Velocity reductions within this region are
interpreted to be associated with low-density, granitic caldera fill,
increased temperatures, higher fracture density and the presence of
hydrothermal fluids, partial melts and magma.

In the northeastern section of the caldera lies Hot Springs Basin, the
largest and most active of the Yellowstone hydrothermal systems. An ~10
km ´ ~20 km, pear shaped low-velocity zone underlines Hot Springs
Basin. Increased fluid pressures supplying Hot Springs Basin as well as
hydrothermally altered rocks at depths inhibit the propagation of
compressional waves, reducing P velocity. Compressional wave velocities
within this zone are as low as 3.4 km s-1 at 4 km depth. NW striking
faults that dip SW towards the caldera extend beneath the Hot Springs
Basin area and coincide with a more diffuse low-velocity zone that
merges with the third low velocity zone, roughly below the Sour Creek
(SC) resurgent dome.

The Sour Creek resurgent dome lacks a strong negative gravity anomaly,
implying density variations at depth are small. Two possible
explanations are consistent with the observed decreased in seismic
velocity. One, a moderate volume of partially melted rock is present
beneath the Sour Creek resurgent dome. Two, hydrothermal fluids
produced by the cooling of partially melted rocks could locally
increase pore pressure and create micorcracks, as is hypothesized for
the Hot Springs Basin anomaly. Miller and Smith (1999) interpreted the
velocity model to imply a local hydrothermal convection cell beneath
the Sour Creek dome that is recharged with meteoric water from within
the caldera. Convection is driven by a region of partial melt and flow
is along the SW dipping faults running thought the region. Velocities
within this ~20 km ´ ~20 km zone are as low as 4.0 km s-1.

A fourth, but less distinct low-velocity zone is present beneath the
southwestern caldera near the Mallard Lake (ML) resurgent dome. The low
P velocities are also interpreted as a zone of partially melted rock
despite an observable gravity anomaly. The smaller size of the Mallard
Lake P velocity anomaly implies a smaller volume of partial melt than
that predicted below the Sour Creek resurgent dome.

The final low-velocity anomaly imaged lies along the Norris-Mammoth
corridor and is not shown in the figure. Miller and Smith (1999) concur
with previous studies (e.g. Lehman et al., 1982) and interpret the
low-velocity zone to be associated with a deep graben bounded on the
west by the Gallatin fault and filled with low-velocity alluvium and
colluvium. In addition, as with all Yellowstone low-velocity features,
fluid or gas saturated sediments associated with hydrothermal systems
in the region are also suspected to contribute to lower observed
velocities.

The seismic structure of the Yellowstone caldera shows low-velocity
anomalies in the upper crust related to Yellowstone's active volcanism.
Within the entire caldera region, P velocities are 1-2 km s-1 lower
than the thermally undisturbed basement rocks immediately in contact
with the caldera. Low velocity, density, and gravity anomalies beneath
the northeast rim of the caldera near Hot Springs Basin imply a
saturated and hydrothermally altered zone characterized by high pore
pressures. Low-velocities within the caldera imply two distinct zones
(Sour Creek and Mallard Lake resurgent domes) of partial melting near
regions of crustal deformation. In the northern section of the caldera
tectonic processes influence crustal velocity structure. The strong
correlation between surface features and subsurface velocity anomalies
leads to a greater understanding of the process that have formed, and
continue to transform, Yellowstone National Park.

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